a. Field of the Invention:
The present invention relates to a focal plane shutter for cameras and, more particularly, to a focal plane shutter having a plural number of shutter blades made of opaque thin plates, said intransparent thin plates being arranged to perform parallel motion by means of a pair of supporting arms in order to open and close an exposure aperture.
B. Description of the Prior Art:
Conventional shutters of this type are arranged as follows. That is, on a base plate having an exposure aperture, base portions of two supporting arms are pivotably mounted at one side of said exposure aperture. A thin plate is pivotably mounted to the free ends of said arms which extend to the other side of said exposure aperture, and said pivots are arranged so that line segments connecting said pivots will form a parallelogram. Said thin plate has dimensions sufficient to cover said exposure aperture. Shutters of the above-mentioned arrangement have various disadvantages for designing and manufacture because they are arranged so that said arms extend across the exposure aperture and said thin plate is supported by the free ends of said arms and is extended toward base portions of said arms so that said thin plate will cover the exposure aperture. For example, when a set of shutter blades is formed by dividing said thin plate into three pieces or more and by pivoting said respective pieces to two arms in the same way as mentioned in the above, the pivot pins for mounting one of the thin plates to the arms become obstacles for mounting the other thin plates. Therefore, a plural number of thin plates should be pivoted to both ends of the arms by turns. Besides, in practice, said arms have to support a plural number of thin plates by their portions for which the amount of movement is comparatively large and, moreover, for which the moving speed is high.
So, said arms should be made comparatively thick. Therefore, if thin plates are pivoted to both ends by turns as described in the above, there will occur a risk of light leakage. Consequently, when a set of shutter blades is arranged by three thin plates, the thin plate in the middle has been connected to arms through intermediate members in order to solve the above problem. In said connecting method, however, construction of the shutter becomes complicated and, moreover, the space occupied by the shutter blade when those thin plates are contracted becomes large because said intermediate members are mounted across said arms. As a result, the shutter as a whole becomes unnecessarily large.
Besides, this type of shutter has two sets of shutter blades comprising plural numbers of thin plates respectively pivoted to two arms as described above and is arranged to complete one exposing operation by opening and closing the exposure aperture as follows. That is, after one set of shutter blades is moved from the extended state to the contracted state, the other set of shutter blades is moved from the contracted state to the extended state. Consequently, at the end of exposing operation, the set of shutter blades operated later sometimes collides against the set of shutter blades which was operated first because of bounding. To prevent such problem, bent portions are provided for the respective main thin plates, which form the exposure slit, out of thin plates for two sets of shutter blades. However, these bent portions cause an increase of the working processes of the thin plates and, moreover, result in increased risk of leakage of light at the end of an exposure. Therefore, it is necessary to make said main thin plates large enough for closing the exposure aperture. As a result, the shutter blade as a whole becomes large and, consequently, the shutter mechanism as a whole also becomes large.